The cause of the weak solar cycle 24

TL;DR

This paper investigates why solar cycle 24 was weaker than previous cycles, attributing it to the emergence of bipolar regions with opposite polarity orientations near the solar equator, which weakened polar magnetic fields.

Contribution

It identifies the emergence of low-latitude bipolar regions with reversed polarity as the main cause of the weak polar fields and the subdued solar cycle 24.

Findings

Weak polar fields caused by low-latitude bipolar regions with opposite polarity.

Emergence of these regions within ±10° latitude significantly impacted cycle strength.

Simulation results support the link between bipolar region orientation and cycle amplitude.

Abstract

The ongoing 11-year cycle of solar activity is considerably less vigorous than the three cycles before. It was preceded by a very deep activity minimum with a low polar magnetic flux, the source of the toroidal field responsible for solar magnetic activity in the subsequent cycle. Simulation of the evolution of the solar surface field shows that the weak polar fields and thus the weakness of the present cycle 24 are mainly caused by a number of bigger bipolar regions emerging at low latitudes with a `wrong' (i.e., opposite to the majority for this cycle) orientation of their magnetic polarities in the North-South direction, which impaired the growth of the polar field. These regions had a particularly strong effect since they emerged within $\pm10^\circ$ latitude from the solar equator.